(1) Field of the Invention
The present invention relates to a method of testing bubble memory devices. More particularly, it relates to a method of detecting defective loops of a magnetic bubble memory device having a plurality of minor loops.
(2) Description of the Prior Art
Before describing a conventional method of testing bubble memory devices, an explanation is given of a general magnetic bubble memory device with reference to FIGS. 1 and 2.
As is illustrated in FIG. 1, a general magnetic bubble memory device comprises a memory chip 2 mounted on an insulating base 1, an XY coil 3, thin plate magnets 4, ferrite yokes 5, pin terminals 6, and a shield case 7. The XY coil 3 applies a rotating magnetic field to the memory chip and effects, for example, the transfer of magnetic bubbles. The thin plate magnets 4 and the ferrite yokes 5 apply a bias magnetic field to the memory chip 2 in order to stably retain the magnetic bubbles in the memory chip. The memory chip 2 comprises a layer of magnetic material formed on a substrate of, for example, GGG (gadlium galium garnet), conductor patterns of, for example, AU or Al-Cu alloy formed on the layer of magnetic material, and transfer patterns of, for example, permalloy, formed on the conductor patterns.
FIG. 2 illustrates a schematic circuit structure of a general magnetic bubble memory device. In FIG. 2, reference numeral 8 designates a bubble generator which comprises a conducting pattern 9 indicated by the broken lines. Reference numeral 10 designates a write-in major line having one end formed on the conducting pattern 9 of the bubble generator 8. The write-in major line 10 comprises, for example, half disc-shaped permalloy patterns. Reference numeral 11 designates a minor loop which is coupled to the write-in major line 10 via swap gates 12 and to a readout major line 13 via replicate gates 14. Reference numeral 15 designates a detector which is coupled to an end of the readout major line 13. The minor loops 11 and the readout major line 13 also comprise, for example, half disc-shaped permalloy patterns.
In the memory device of FIG. 2, when the writing in of bubble signals is effected, the bubble signals generated by the bubble generator 8 are transmitted through the write-in major line 10, in series, by a driving magnetic field from the XY coils. When magnetic bubbles of the bubble signals are transmitted to the corresponding bit positions of the write-in major line 10 associated with the positions of the minor loops 11, a swap pulse is applied from a swap-conducting pattern 16 to the swap gates 12, thereby transferring, in parallel, the bubble signals on the write-in major line 10 to the minor loops 11.
When the reading out of the bubble signals is effected, a replicate pulse is applied from a replicate-conducting pattern 17 to the replicate gates 14, and magnetic bubbles stored in the minor loops 11 adjacent to the replicate gates are transferred, in parallel, to the readout major line 13. The magnetic bubbles on the readout major line 13 are transmitted to the detector 15, in series, and are converted into readout signals which are output from output conductors 18.
With reference to FIGS. 3 through 5, a conventional method of testing bubble memory devices is explained.
The characteristics of a bubble memory device are shown in FIG. 3, in which the driving magnetic field HD is plotted on the abscissa and the bias magnetic field HB is plotted on the ordinate. The margin curve 20 is drawn, and, ordinarily, the hatched range is an operation region. At delivery inspection in a manufacturing plant or the like, instead of drawing the margin curve 20 to determine the operation region, there is adopted a window test method in which, as is shown in FIG. 3, the driving magnetic field and bias magnetic field are changed to several points (1), (2), . . . on the coordinates X and Y to check the operation within a region defined by these points, whereby the test time is shortened. In devices having a major-minor loop structure or a block replicator structure; that is, in bubble memory devices having a plurality of minor loops, which are widely used at present, if there are some defective minor loops, precise detection of the defective loops occupies a major portion of the test time. When a defective loops is detected, this is displayed on the device so that the loop is not used and only the normal minor loops are used; thus, increasing the yield.
Whether or not a minor loop is defective depends on the driving magnetic field and the bias magnetic field and on the bubble information pattern within minor loops interreaction occurs among bubbles. The test apparatus shown in FIG. 4 is ordinarily used for the detection of defective loops. In FIG. 4, reference numeral 22 designates a bubble memory device, reference numeral 24 designates a control circuit for a bias magnetic field applied to the device 22, and reference numeral 26 designates a driving circuit for coils X and Y for a rotating magnetic field applied to the device 22. Reference numeral 28 designates a system control circuit, reference numeral 30 designates a circuit for generating a write information pattern, and reference numeral 32 designates a gate pulse driving circuit. By using these elements, predetermined bubbles are generated and are driven by the rotating magnetic field generated by the coils X and Y. Refernce numeral 34 designates a circuit for detecting readout information, and reference numeral 36 designates a comparing circuit for comparing the output of the pattern-generating circuit 30 (that is, the information written into the device 22), with the output of the detecting circuit 34; that is, the information read out from the device 22. When these two outputs coincide with each other, the device 22 is judged as being normal. When they do not coincide with each other, the device 22 is judged as being abnormal, and the defective minor loop is detected from the position of the bit where the discrepancy appears.
This discrepancy, that is, a propagation error, can be generated by various factors. These factors include, for example, whether or not the bubble generator precisely generates bubbles; whether or not the bubbles are precisely transferred into the minor loops from the major loop and the writing gate, i.e., the swap gate; whether or not the bubbles are normally transferred among the minor loops; whether or not the readout gate, i.e., the replicate gate, prcisely reads the bubbles from the minor loops; and whether or not the major loop precisely transfers these bubbles to the bubble detector. The respective intensities of the bias magnetic field HB, and of the driving magnetic field HD are related to these factors, and certain portions of the device (loops, gates, and so on) operate normally under certain combinations of HB and HD [conditions (1), (2), . . . in FIG. 3], but these portions operate erroneously under other combinations of HB and HD. Accordingly, in the conventional sequence for detection of detective loops, writing, transferring, reading, and comparing are carried out for every driving magnetic field and bias magnetic field condition, as is shown in FIG. 5.
More specifically, information to be written is first generated by the bubble generator, and the transfer gates or swap gates are actuated to transfer this information to the minor loops (writing in). Then, bubbles (written information) are propagated in the minor loops. In this case, start-stop operations (intermittent driving) are ordinarily carried out simultaneously so that the bubbles pass through all of the bits in the loops (minor loop transfer). Then the transfer gates or block replicators are actuated to transfer the bubbles to the major loop, and the corresponding signals are read out by the detector (readout). Finally the written information is compared with the readout information, and the loop where the written information and the readout information do not coincide with each other is judged as being defective.
The number of minor loops indicates the number of bits in a page and the bit number within the minor loop indicates the page number. In the above example, the latter number is 2,048; that is, the number of bits in each minor loop. Bubbles corresponding to the number of bits in a page are generated, and the writing and reading of these bubbles are repeated a number of times corresponding substantially to the number of pages (conditions are most strict when bubbles are stored in all of the pages). An example of the simplest test sequence is shown in FIG. 5. Practically, however, a more complicated test sequence is used. In a typical practical test sequence, the reading operation is repeated, or, after the writing sequence minor loop transferring and reading are repeated several times while changing the write information pattern, writing, reading, and comparing information operations are carried out.
When a detective loop is detected according to the conventional method as was pointed out above, the defect depends greatly on the driving magnetic field, the bias magnetic field, and the bubble information pattern within the minor loop. Therefore, after bubble information is stored in substantially all of the pages and every point of the driving magnetic field and the bias magnetic field shown in FIG. 3 is exercised per the sequence of FIG. 5, it is necessary to take the logic sum of the defective loops detected at the respective points and indicate the defective loops of the bubble memory device. For example, in the case where a 1-megabit device is tested at a driving frequency of 100 KHz, about 12 seconds (10 .mu.S.times.600.times.2,048) are necessary for writing, about three seconds are necessary for the minor loop transfer inclusive of the stopping and starting operations, and about 12 seconds (10 .mu.S.times.600.times.2,048) are necessary for reading. Therefore, even if the window test shown in FIG. 3 is executed using the simplest sequence, 216 seconds {(12+3+12).times.8 } are required to test the device.